Heater vents

Today a man from the utility company came to do an energy efficiency check on our house. He put in 10 CFL's, replaced the showerheads, and checked for links in the heating ducts. Something about the whole process stirred a childhood memory that I want to share here.

When we lived in the white house in Michigan, there was a heating vent in the living room under an end table. When the heat came on, I (and my siblings) liked to crawl under the table and just soak up the hot air from the heater. We also liked to put coats over the vent to get them nice and toasty warm before we went outside. Mom allowed this, but only on a small scale. every so often, she'd come in and tell us that we had to move away from the vent because if we kept it covered, the hot air would never reach the thermostat and the furnace would stay on indefinitely. As a child, I could never figure out why this was a bad thing. I liked sitting by the heater when it was on, and sitting by the heater made it stay on, so why should I move and let it turn off?